1. Field of the Invention
The present invention relates to a motor control device that controls, for example, a brushless motor used in an automatic transmission mounted on a vehicle, such as an automobile.
2. Description of the Background Art
Generally, a vehicle, such as an automobile, equipped with an automatic transmission uses an engine as a power source and is enabled to run by transmitting motive power generated by the engine to wheels via the automatic transmission. A start clutch is provided inside the automatic transmission and the start clutch is engaged when motive power transmitted from the engine is transmitted to a gear mechanism. Conversely, the start clutch is disengaged when motive power transmitted from the engine is not transmitted to the gear mechanism. It is configured for the start clutch to be engaged and disengaged under the control of a clutch control portion.
In case an engaging force of the clutch becomes unstable, motive power transmitted from the engine to the gear mechanism becomes unstable and eventually motive power transmitted to the wheels becomes unstable. A running condition of the automobile thus becomes unsteady, which makes the driver feel uncomfortable. Hence, in order to ensure a steady running condition of a vehicle, it is necessary to control an engaging force of the start clutch with accuracy.
Incidentally, there is an automatic transmission that transmits motive power from the engine to the gear mechanism using the start clutch as described above. In this automatic transmission with the start clutch, an actuator is equipped to a dry-type single-plate start clutch so that a clutch engaging force is adjusted by varying a stroke amount of the start clutch using the actuator. For a mechanism in which a motor is used as the actuator and a rotational angle of the motor is proportional to a stroke amount of the start clutch, it is necessary to adjust an amount of torque of the motor with accuracy in order to adjust an engaging force of the start clutch.
Because an amount of torque of the motor is proportional to an amount of current of the motor, it is necessary to enhance accuracy of current control of the motor in order to adjust an amount of torque of the motor with accuracy. There is a motor control device that controls an amount of torque of the motor by applying feedback control to the current control of the motor. It is, however, crucial for such a motor control device to detect a current of the motor with accuracy.
A motor current detection method is proposed, for example, in Patent Document 1. This is a method of suppressing a variance of a detection current value in a control device of a brushless motor of a 120° rectangular wave conduction method by detecting a current at timing at which a motor current reaches a peak value when switching ON-and-OFF patterns (hereinafter, referred to as the conduction patterns) of respective FETs (Field Effect Transistors) forming a motor drive circuit in synchronization with edge signals of multiple hall sensor signals used to detect a position of a rotor with respect to a stator of the brushless motor.
Another technique of detecting a motor current is proposed, for example, in Patent Document 2. According to this technique, a current detected in every current detection cycle is approximated to a straight line and a motor current at a predetermined timing is calculated by a correction calculation using the current detection cycle and a time taken until an applied voltage is actually applied to the motor.
[Patent Document 1] JP-A-2009-281538
[Patent Document 2] JP-A-2011-78291
When the technique disclosed in Patent Document 1 is applied to the motor control device in the related art, because a command voltage is kept applied since a current detection timing till a conduction pattern switching timing, a motor current rises from the current detection timing to the conduction pattern switching timing. Hence, an error occurs between a detection current value detected at the current detection timing and the peak value of the motor current that is originally supposed to be measured. Accordingly, there is a problem that when the detection current value is applied to the feedback control, an error occurs between a motor target current and the peak value of a current actually flowing to the motor.
This problem of the technique in the related art will be described in detail with reference to FIG. 6. FIG. 6 shows a motor current over time in a case where a motor current is detected when conduction patterns of an inverter are switched in synchronization with an edge signal of a hall sensor signal generated at a predetermined motor electrical angle, and a command voltage is outputted by applying the current value thus detected to the feedback control. Referring to FIG. 6, Im11 through Im13 indicate a detection current value detected at the current detection timing, Ip11 through Ip13 indicate a motor current value at the conduction pattern switching timing, and Vm11 through Vm13 indicate a command voltage. Assume that a motor electrical angle advances in periods a, b, c, and d by 60 degrees per period.
By applying the detection current values Im11 through Im13 detected as above to the feedback control, the detection current values Im11 through Im13 are allowed to follow a motor target current (indicated by a dotted line of FIG. 6). However, the command voltage is kept applied for a time from the current detection timing to the conduction pattern switching timing. The motor currents at the conduction pattern switching timing therefore rise to Ip11 through Ip13. Consequently, errors Ie11 through Ie13 occur between the peak values Ip11 through Ip13 of the motor current and the motor target current, respectively.
Also, when a correction calculation is performed using a current detection cycle T11 and a time T12 taken since the current is detected till an applied voltage is actually applied to the motor as in the technique disclosed in Patent Document 2, a result of the correction calculation is limited to a value on a straight line linking the detection current values Im11 through Im13. Hence, the peak values Ip11 through Ip13 of the motor current cannot be calculated.
As has been described, the related art has a problem that an error occurs between the peak value of the motor current and the motor target current because the motor current detected at the current detection timing keeps rising till the conduction pattern switching timing.
Generally, a motor current and an amount of torque of the motor have a proportional relation. Hence, in a case where an error is occurring between the motor current and the motor target current, an error also occurs in an amount of torque of the motor. As has been described above, in order to control a clutch engaging force with accuracy, it is necessary to adjust an amount of torque of the motor with accuracy. Hence, when an error occurs in an amount of torque due to an error occurring between the motor current and the motor target current, there arises a problem that a clutch engaging force cannot be controlled with accuracy. A limit value of error accuracy of the motor current is thus determined by control accuracy of a clutch engaging force.